As shown in Japanese Patent Publications Kokai H1-315041 and H1-315040, a conventional optical head system for an optical disk reproducing apparatus includes a semiconductor laser, a collimator lens, a phase shift plate or a shade plate for realizing super resolution, first and second polarization beam splitters, a 1/4 wavelength plate, an object lens, an information signal detector and an error signal detector.
In operation, a laser beam generated by the semiconductor laser is supplied through the collimator lens, the phase shift plate, the first and second polarization beam splitters, the 1/4 wavelength plate and the object lens to the optical disk. A light reflected from the optical disk is backwardly supplied through the object lens and the 1/4 wavelength plate to the second polarization beam splitter so as to guide the reflected light to the information signal detector. In accordance with the light detected by the information signal detector, information recorded on the optical disk is reproduced. A light which has passed through the second polarization beam splitter is reflected from the first polarization beam splitter to the error signal detector so that error signals are detected thereby.
Although the focal length of the object lens and the distance between the phase shift plate (or shade plate) and the object lens should be the same in order to equalize phase distribution of a condensed light spot on the optical disk, however, it is difficult to meet such a requirement according to the above mentioned conventional system, because the first and second polarization beam splitters and the 1/4 wavelength plate are arranged between the phase shift plate and the object lens.
Although the phase shift plate and the object lens must be aligned precisely on the common axis in order to have a light spot having symmetric strength distribution on the optical disk, however, according to the conventional optical head system, the object lens must be moved to control tracking and whereby the optical axis shifts.